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Angewandte
Chemie
Platinum Complexes
The Role of Substituent Effects in Tuning Metallophilic Interactions
and Emission Energy of Bis-4-(2-pyridyl)-1,2,3-triazolatoplatinum(II)
Complexes**
M. R. Ranga Prabhath, Julia Romanova, Richard J. Curry, S. Ravi P. Silva, and
Peter D. Jarowski*
Abstract: The photoluminescence spectra of a series of 5-
substituted pyridyl-1,2,3-triazolato PtII homoleptic complexes
show weak emission tunability (ranging from l = 397–408 nm)
in dilute (10À6 m) ethanolic solutions at the monomer level and
strong tunability in concentrated solutions (10À4 m) and thin
films (ranging from l = 487–625 nm) from dimeric excited
states (excimers). The results of density functional calculations
(PBE0) attribute this “turn-on” sensitivity and intensity in the
excimer to strong Pt–Pt metallophilic interactions and a change
in the excited-state character from singlet metal-to-ligand
charge transfer (1MLCT) to singlet metal-metal-to-ligand
charge transfer (1MMLCT) emissions in agreement with
lifetime measurements.
properties and allow for a more minimal planar structural
motif with less diasteromeric diversity. However, anticipated
deleterious excitonic self-quenching of planar complexes
brought on by enhanced intermolecular electronic coupling
through p–p stacking interactions implies considerable design
difficulties for this class of phosphors. Nonetheless, for
PtII complexes this negative aspect is largely offset by the
propensity of these systems to engage in metallophilic Pt–Pt
interactions. These interactions give rise to new and interest-
ing photophysics involving transitions between metallophilic
bonds and ligands, denoted metal-metal-to-ligand charge
transfer (MMLCT), providing supramolecular design poten-
tial for further property tunability.[6] Indeed, a number of
studies have shown strong emission intensity despite clear
metallophilic stacking.[7] A better understanding and control
of such interactions may lead to materials with increased
quantum yields and potential anisotropic ordering arranged
through discrete noncovalent interactions. Further studies of
the excimer states of such systems through systematic
alteration of the ligands would reveal important design
criteria towards these goals.
In this study the photophysical properties of novel
pyridyltriazolate complexes of platinum(II) are explored.
Compared to other pyridyl azoles,[7a,8] pyridyl-1,2,3-triazoles
are less studied. Moreover, most of the examples related to
pyridyl-1,2,3-triazoles involve substitution at the N atom of
the triazolyl ring, which should affect the energy of the
highest occupied molecular orbital (HOMO). However,
triazole is electronically insulating and thus substituent effects
have not been very pronounced.[9] A more effective tuning
route might target the lowest unoccupied molecular orbital
(LUMO) that is located on the pyridyl ring and is anticipated
to be more closely associated with the excited state.[8b] Thus, in
the present work, pyridyl-1H-1,2,3-triazole ligands were
prepared using efficient reactions to build a structurally
homologous series of donor and acceptor 5-pyridyl-substi-
tuted anionic ligands. These ligands were prepared by
deprotonation of the 1H-triazolyl moiety to give strong-field
ligation (suppressed d–d transitions)[10] in the neutral final
complexes (Scheme 1). Keeping synthetic efficiency in mind,
the target ligands were approached through the serial
application of the Sonogashira carbon–carbon coupling[11a]
and the Sharpless copper-catalyzed Huisgenꢀs 1,3-dipolar
cycloaddition procedures (see the Supporting Information,
Sections S1 and S2 for experimental procedures).[11b] Starting
from the 2-bromopyridyl derivatives 1a (R = NMe2), 1b (R =
H), and 1c (R = CHO), Sonogashira coupling with trimethyl-
O
wing to superior energy efficiency, light emitting diode
(LED) technology has become considerably commercialized
over the last decade.[1] For organic LEDs (OLEDs), innova-
tions have been spurred along by the discovery of new
molecules with good stability and high emission intensity,
followed through by intense engineering efforts. Heavy-
metal-containing complexes are potent molecular emitters
as a result of their high quantum efficiencies (QEs, f) related
to facile intersystem crossing (ISC) between excited-state
manifolds (efficient spin orbit coupling (SOC)) and resultant
efficient emission from the triplet state (phosphorescence).[2]
Rational tuning of the emission wavelengths based on
structural modifications of the ligands,[3] with concomitant
control of the QE, has been demonstrated in a number of
octahedral complexes of d6 metals[4] including predominantly
iridium,[4a,b] rhodium,[4c] and ruthenium[4d] and with a large
range of structurally diverse ligands. The square-planar d8
complexes have also received considerable interest since the
seminal work of Gray, Vlcek, Miskowski, and co-workers.[5]
Square-planar platinum complexes present excellent emissive
[*] M. R. R. Prabhath, Dr. J. Romanova, Prof. R. J. Curry,
Prof. S. R. P. Silva, Dr. P. D. Jarowski
University of Surrey, Advanced Technology Institute
Guildford, GU2 7XH (UK)
E-mail: p.d.jarowski@surrey.ac.uk
[**] We would like to thank the EPSRC (EP/K009664/1) (UK), a Royal
Society Research Grant (RG11041), and the Leverhulme Trust (RPG-
2014-006) for funding as well as the National Service for
Computational Chemistry Software (NSCCS) at Imperial College
London and the National MS Service. M.R.R.P. would like to thank
the Southeast Physics Network (SEPnet) for student funding.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2015, 54, 7949 –7953
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7949